Light-emitting diodes (LEDs) of different color have different forward voltages as they require different excitation to produce light output. Therefore, LED strings of different color can't be driven with a single direct current (DC) or alternating current (AC) bus voltage. Also, due to the non-linear I-V (current-voltage) characteristics and negative temperature coefficient, LEDs have to be driven with constant current sources.
Traditionally, LED lighting systems use either single- or multiple-string phosphor-converted white LEDs to deliver the desired lumen level. The phosphor-converted white LED lighting systems with multiple LED strings may have different forward voltages due to temperature variations, non-linear characteristics of LEDs, and poor binning in LED manufacturing. LED drivers that drive multiple LED strings from a single voltage or current source require current balancing mechanisms for constant current regulation in each LED string. Passive and active approaches have been used to counteract voltage mismatch in LED strings and to provide constant currents.
The passive approaches are based on using passive elements, such as capacitors, transformers, and inductors, in different configurations. Capacitor-based current balancing mechanisms underutilize the LEDs and also depend on the tolerance of the capacitances used. Transformer and inductor passive approaches are bulkier in size.
The active balancing approaches are either based on using conventional linear current regulators, such as current mirrors, or by using individual power converters to drive each of the LED strings. In the case of linear current regulators, the efficiency of the converter decreases with the increase in the voltage mismatch between the LED strings. Individual switched mode regulators providing constant current to each LED string are more efficient but are not cost effective due to their higher component count.